Crank seals and head gaskets

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#1
88 KX250
My bike started blowing anti freeze out the over flow. Removing the head revealed a leaking head gasket. The bike had been sleeved and the sleeve was actually taller than the rest of the cylinder. I did a poor man's surfacing job. I glued 600 grit paper to a stack of three 1/4" pieces of glass and wet sanded the cylinder and head until there was a consistent pattern. The head and cylinder are now straight but the piston at TDC just touches the head without a head gasket. What kind of clearance should I have between the piston and head? Should I increase the dome of the head or chamfer the piston? My crank seals also started leaking so I'm replacing them. Anyone have some tips on how to install the crank and bearings without throwing the crank out of alignment? I have a press.
 
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#2
Originally posted by shueman
What kind of clearance should I have between the piston and head? Anyone have some tips on how to install the crank and bearings without throwing the crank out of alignment? I have a press.
shueman.

Not sure what the recommended clearance is but, you should have enough when you add the head gasket.

In regard to installing the crank. DON'T USE A PRESS!
Something that works pretty good is to install both main bearings on the crank and seat them all the way down. Then take a torch and evenly heat one half of the engine cases. When the case is heated to the point where a drop of water will sizzle against the case, take the crank and drop it in the case. If the crank does not go all the way in, take it back out and reheat the case and do it again. Don't force it or press it in.

After that case has cooled down, apply your gasket sealer and heat the other case half. When it is hot enough, carefully and evenly set it down over the crank and gear shafts and using a rubber, plastic or rawhide hammer, tap the cases together. If they don't go all the way together or bind in any way, push them back apart and do it again.

Be very careful when heating the cases, it doesn't take very much heat to melt aluminum. If you don't have a torch, you can use an oven. The only drawback to using an oven is making up a good excuse for when the lady of the house asks, " Why do my chocolate chip cookies taste like two stroke oil?" :confused:

Just my $ .02
 
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#3
Thanks.
I've heard and read so many different ways to do the crank. Your way seems the most logical and reduces any chance of messing up the crank. I'll clean the cases with solvent then soapy water. It's actually my Mom's oven. She can't get mad at her son right?
I'll check the head clearance with the gasket and some clay.
 

kawdude

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#4
Originally posted by Ol'89r


In regard to installing the crank. DON'T USE A PRESS!
Something that works pretty good is to install both main bearings on the crank and seat them all the way down. :confused:

Just my $ .02
Whats the best way to push the bearings on the crank
 
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#5
Originally posted by kawdude


Whats the best way to push the bearings on the crank
kawdude.

Not sure what is the best way but, this is how I do it.

Find a piece of steel plate that will fit between the crankshaft halves. Lay the crank on the steel plate as to support the back side of the crank half. Then take a deep socket that will fit over the shaft part of the crank and is the same diameter as the INNER race of the bearing. Using the socket and either a hammer or a press, ( you can use a press for this), drive the bearing all the way down until it seats solid against the crank half. Flip the crank over and do the other side. Be sure to use a little assembly lube on the shaft and bearing and never push against the outside race of the bearing. This can damage the bearing.

Hope this helps. :thumb:
 

kawdude

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#6
Just so I understand...

The plate is between the crank halfs and supporting the half that I would be putting the bearing on. Pushing against the one side that I'm placing the bearing on would not effect trueness of the crank.

Do you recommend freezing the crank and heating heating the bearings?
 
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#7
Originally posted by kawdude
Just so I understand...

The plate is between the crank halfs and supporting the half that I would be putting the bearing on. Pushing against the one side that I'm placing the bearing on would not effect trueness of the crank.

Do you recommend freezing the crank and heating heating the bearings?
kawdude.

That is exactly right. Only push against one side at a time.

You can freeze the crank and heat the bearing to make it a little easier but, you have to be careful to have plenty of oil on the crank big end or the moisture from freezing could cause rust in the bearing.

Most cranks only have a slight interference fit on the main bearing. Usually only a little amount of assembly lube or anti-seize compound is required for the bearing to slip down over the shaft.

It is also a good idea to use heat when spliting your engine cases and removing the crank. Heating the case expands the aluminum and allows the main bearing to fall out of the case rather than be pushed out and take a little of the aluminum with it. Every time you push a bearing out and back in without expanding the case first, it wears the aluminum surface where the bearing goes in and eventually will cause the bearing to be loose in the case.
 

kawdude

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#8
I'm starting to reassemble my engine and have a question about installing the lower seals and bearings. Early I heated the case in the oven to 300 degrees for 12 minutes and the bearings would not drop into the case.

How hot and how long should the case be heated for the bearings to drop in.

Will this effect the rubber on the seals?

Also, how do I know the seals are all the way in? I put mine in and they appear to have bottomed but there is still some sticking up on the inside of the case. Is this normal? My concern is that the part of the seal on the inside is partially blocking the oil hole from the cylinder.